The present invention relates to perforating guns and particularly to a perforating gun assembly that is designed to be run through the tubing string of a well. In particular, the assembly is designed for use in deep wells with high temperatures and high pressures. Temperatures would normally be in excess of 340.degree. F.
In the production of hydrocarbons, both crude oil and natural gas, it is customary to drill a well and install casing if it appears that commercial production can be realized. After the well is cased, it is completed by perforating the casing opposite the production zones. The perforating is done by explosive charges which fire a high energy jet through the casing and into the formation to create drainage paths for the hydrocarbons. In the case of low temperature and relatively low pressure wells, the wells can be perforated using a string of perforating charges that is lowered through the casing and actuated to perforate the casing. The string of expended charges and supporting structure can then be withdrawn from the well and the production tubing string installed and the well produced. In the case of deep wells, it has been customary to first install the production tubing string and then perforate the well by a string of charges that are lowered through the production tubing string. This can be done by either using expendable guns or a hollow carrier, which carries the charges, that can be retrieved. In the case of expendable guns that are lowered through the production tubing string, it is necessary to separate the charge carrier from the wire line used for lowering the charges in order that the carrier may be left in the well bore. Normally, the carrier is designed to fracture into small pieces when the charges are detonated and the pieces fall to the bottom of the well. The system of using charges suspended on a carrier is satisfactory in wells that have temperatures below approximately 300.degree. F., but cannot generally be used in wells above 300.degree. F., due to the temperature effect on the primacord and detonator used in the perforating gun. For higher temperatures a retrievable gun is used, but the hollow carrier is designed with sufficient strength and wall thickness to limit swelling beyond a pre-determined amount. This allows the gun carrier to be retrieved. The size of the carrier of this system limits the charge size and potential perforation penetration (effectiveness).